Hydrocarbon class separation and quantitation by split column effluent analysis

ABSTRACT

Methods of separating and classifying the relative amounts of two or more classes of hydrocarbon molecules in a sample are disclosed wherein an input stream comprising a mobile phase and the sample is injected into a chromatographic column, and an effluent stream exiting a chromatographic column is split into a first effluent stream and a second effluent stream. The mass of carbon in each of the classes of hydrocarbons in the first effluent stream is determined, preferably using a flame ionization detector, and the second effluent stream is directed through a variable restrictor, allowing the pressure and mass flow rate of the input stream to be independently controlled and improved results obtained. In a preferred embodiment the present invention also includes identifying the hydrocarbon molecules in the second effluent stream in order to verify the analysis or provide additional data, preferably by using an ultraviolet detector. The present invention is most preferably used to analyze the aromatic hydrocarbons in a sample of diesel fuel.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This is a divisional of application Ser. No. 08/026,385 filed on Mar. 4,1993, now U.S. Pat. No. 5,346,622.

The present invention relates to analytical chemistry, and inparticular, the present invention provides methods and apparatus forseparating and quantifying classes of hydrocarbons in a sample.

BACKGROUND OF THE INVENTION

A number of chromatographic techniques are known for separating thecomponents of a sample. In packed column chromatography, a mobile phasefluid and a sample flow through a column containing a stationary phase,which is chosen to retain specific components of the sample.Supercritical fluid chromatography (SFC) uses a supercritical fluid,usually carbon dioxide, as the mobile phase. The solvent power of themobile phase in supercritical fluid chromatography is a linear functionof density, which is in turn related to pressure. However, in systemssuch as SFC systems that use compressible fluids at pressures aboveatmospheric (ambient), the pressure of the system is coupled to the massflow rate.

The problems caused by the coupling between pressure and mass flow ratein supercritical fluid systems and their solution are explained indetail in commonly-assigned U.S. Pat. Nos. 5,133,859; 5,094,741 and U.S.patent application Ser. No. 804,155 filed Dec. 6, 1991—Frank et al.,which are all incorporated herein by reference. Frank et al. teach thatmass flow rate and pressure can be “decoupled” and independentlycontrolled by providing a variable orifice restrictor downstream from,e.g., the column of a chromatographic instrument or the extractionchamber of an-extraction instrument. Variable orifice restrictors andtheir use are disclosed in commonlyassigned U.S. Pat. Nos. 5,009,778;5,151,178 and 5,178,767—Nickerson et al. which are all incorporatedherein by reference. As used herein, “variable restrictors” refer tovariable orifice restrictors such as those disclosed in the Frank andNickerson patents, as well as other types of restrictors in which thedegree of restriction is changed by varying the size of an orifice,occluding an orifice, changing other physical characteristics of anorifice or interchanging and selecting fixed orifices of varying sizeswithin a valve or similar device.

A specific application of supercritical fluid chromatography is theanalysis of the aromatic content of diesel fuels using packed columnsand a flame ionization detector (FID). The test method is set forth inASTM D5186-91, published by the American Society for Testing andMaterials, 1916 Race Street, Philadelphia, Pa. (USA) 19103-1187, whichis incorporated herein by reference. The above-referenced ASTM method isgenerally applicable to many different types of petroleum and chemicalsamples where the goal is to determine the relative amounts of differentclasses of hydrocarbons. Basically, saturated hydrocarbons are separatedfrom aromatic hydrocarbons in an SFC column and the flame ionizationdetector provides a signal based on the mass of carbon in each class ofhydrocarbon.

The ASTM method specifies a fixed restrictor at the end of the SFCcolumn to maintain the necessary pressure for supercritical fluidchromatography. However, there are several disadvantages to thisconfiguration. First, fixed restrictors clog, changing retention timeand the response of the flame ionization detector. Also, since everyrestrictor is different, no two sets of analysis equipment will haveequivalent retention times, thereby introducing an uncertainty into thetest method. Additionally, as explained above, in a fixed restrictorsystem, flow rate through the column is directly related to the pressureand the restrictor, so optimization and maintenance of chromatographicconditions is very difficult.

Another problem with the ASTM method is that the response of flameionization detectors becomes nonlinear and non-uniform for the differentclasses of hydrocarbons as the flow the supercritical fluid mobile phasebecomes too large. This limits the size of columns that can be used.Typically, the larger the column size, the larger the flow to the flameionization detectors. Thus, in this type of system, smaller columns areless problematic than larger columns. It would be desirable to be ableto control the flow rate to permit the use of columns having a largerinner diameter than typical SFC columns.

Finally, even though the flame ionization detector provides a signalrepresenting the mass of the carbon atoms in the molecule, some dieselfuels with relatively the same amount of aromatic compounds performquite differently in engines. Therefore, the flame ionization detectorsignal alone is not sufficient to differentiate all the diesel fuelsbeing tested. Accordingly, it would be further desirable to provide asystem wherein additional analytical techniques or instruments can beincorporated to simultaneously analyze the effluent stream from the SFCcolumn to more accurately characterize the sample.

SUMMARY OF THE INVENTION

It has now been found, however, that the shortcomings of the prior artmethods of separating and classifying the relative amounts of two ormore classes of hydrocarbon molecules in a sample can be overcome. Thepresent invention discloses methods wherein an input stream comprising amobile phase, preferably a supercritical fluid, and the sample isinjected into a chromatographic column at predetermined conditions ofpressure and flow rate, and an effluent stream exiting thechromatographic column is split into a first effluent stream and asecond effluent stream. Preferably, first effluent stream has asubstantially smaller mass flow rate than the second effluent stream,and most preferably, comprises less than 5% of the input stream. Bydetermining the mass of carbon in each of the classes of hydrocarbons inthe first effluent stream, preferably using a flame ionization detector,and directing the second effluent stream through a variable restrictor,the pressure and mass flow rate of the input stream can be independentlycontrolled while improved results are obtained. In a preferredembodiment the method includes the further step of identifying thehydrocarbon molecules in the second effluent stream in order to verifythe analysis or provide additional data. Preferably, the second effluentstream is directed into a detector, such as an ultraviolet detector, forthis purpose. In a most preferred embodiment, the methods of the presentinvention are used with a sample comprised of diesel fuel, and theclasses of hydrocarbons comprise saturated and aromatic hydrocarbons.

The present invention also discloses preferred embodiments of apparatusfor separating and classifying the relative amounts of two or moreclasses of hydrocarbon molecules in a sample in accordance with theabove-described methods. Preferably, a chromatograph for producing aneffluent stream, a detector receiving a first portion of the effluentstream, and a variable orifice restrictor receiving a second portion ofthe effluent stream are provided. The variable restrictor permits theindependent control the pressure and flow rate of the input stream. Thechromatograph is most preferably a supercritical fluid chromatographthat includes a column packed with an adsorbent such as silica; thecolumn most preferably has an internal diameter of between 1 to 5millimeters (mm). In certain embodiments, an apparatus for identifyingthe hydrocarbon molecules in the second effluent stream, such as amulti-wavelength ultraviolet detector is also included.

The variable orifice restrictors used in the present invention may be ofany of a number of different types. For example, the restrictor maycomprise an axially moveable pin to restrict mass flow by varying thesize of an orifice. Mass flow rate may also be varied by selecting anorifice plate comprising a fixed orifice and inserting the orifice plateinto a variable restrictor device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the apparatus of the presentinvention.

FIG. 2 is a schematic representation of an alternate embodiment of theapparatus of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is shown a schematic representation of anapparatus made in accordance with the present invention. Typically, asource of the mobile phase fluid, such as a pump 100 is connected to aninjection valve 102 for introducing a sample 50 to be analyzed into achromatographic column 104. The flow into the column 104 forms an inputstream, that flows through the chromatographic column 104, which ispacked with a stationary phase (not shown), that is most preferablycomprised of an adsorbent such as silica and the sample is eluted by themobile phase. Those of ordinary skill will understand that the column104 requires surrounding instrumentation and controls and will typicallybe resident within a chromatograph instrument, such as the HP G1205A,manufactured by Hewlett-Packard, Palo Alto, Calfi. (USA). However, itwill also be understood that other suitable analytical instruments forisolating aromatic compounds or other classes of hydrocarbons can beused in conjunction with the present invention.

After the input stream has flowed through the chromatographic column104, an effluent stream will be created. In the present invention, theeffluent stream is split into at least two streams at a tee 106, asshown in FIG. 1. It will be appreciated that the effluent stream canalso be split into multiple (e.g. three or more) streams in certainembodiments. In the embodiment illustrated, a first effluent stream isconnected to a flame ionization detector 110 and, as explained above,produces a signal indicative of the mass of carbon in each class ofhydrocarbon compounds in the sample. As known to those of skill in theart and specified by the ASTM method D5186-91 described above andincorporated herein by reference, the inlet to the flame ionizationdetector 110 includes a fixed restrictor 112. A second effluent streamflows from the tee 106 to a variable restrictor 120, which permits thepressure and flow rate in the system to be “decoupled” and separatelycontrolled in accordance wit the present invention. In addition topermitting pressure and flow rate to be separately controlled, thevariable restrictor 120 increases retention time reproducibility becauseit is adjustable and relatively immune to clogging, unlike the fixedrestrictor specified by the ASTM method which clogs, causing changes inretention time.

The benefits of the present invention are derived from the splitting thecolumn effluent into two streams. Preferably, the two streams are notequal, and the fraction of the effluent which flows to the flameionization detector 110 is a function of the pressure at the exit of thecolumn and the resistance of the fixed restrictor 112 at the inlet ofthe flame ionization detector 110. Preferably, the effluent flow to theflame ionization detector 110 is small relative to the flow continuingto the variable restrictor 120. In a most preferred embodiment, lessthan five percent (5%) of the flow exiting the column 104 is directed tothe flame ionization detector 110. By splitting the effluent steam theflow rate to the flame ionization detector 110 is reduced, and thus moreaccurate response for different hydrocarbon classes is obtained.Additionally, even if the fixed restrictor 112 that must be used withthe flame ionization detector 110 becomes clogged, the split effluentconfiguration makes the analysis more reliable because the majority ofthe effluent flow is directed to the variable restrictor and theretention time of the peaks are insignificantly affected.

In preferred embodiments of the present invention, the chromatographiccolumn 104 is larger than the columns typically used in supercriticalfluid chromatography and are preferably the standard columns used forhigh pressure liquid chromatography (HPLC). Splitting the effluent steamallows such larger diameter columns to be operated at optimum flowrates. The column 104 used in the present invention most preferablyinner diameter between approximately 1 to 5 millimeters (mm). The largercolumns are more rugged and have longer lifetimes, and are packed moreefficiently than smaller size columns typically required forsupercritical fluid chromatographs that direct all of the column flow tothe flame ionization detector 110. Although larger columns have moreinherent benefits, 1 mm columns are commonly used in supercritical fluidchromatography and are also useful with the present invention.

Referring now to FIG. 2, another advantage created by splitting theeffluent stream can be seen. As shown in FIG. 2, a multi-wavelengthultraviolet (UV) detector 130 can be incorporated into the system of thepresent invention. This device allows the simultaneous verification ofthe accuracy of integration of the flame ionization detectorchromatogram and differentiation between samples of similar aromaticcontent. In addition to the multi-wavelength detector 130 described withrespect to FIG. 2, it will be appreciated that other types of analyticaldevices could be incorporated into the system of the present inventionin a similar matter. For example, an infrared detector or a refractiveindex detector could be used to identify the constituents of theeffluent stream to correlate the results obtained from the flameionization detector 110. Also, as mentioned above, additional streams ofeffluent may be formed in certain embodiments; in the case where one ofthese additional “splits” is formed before the variable restrictor 120,this additional stream would be analyzed using a mass spectrometer.

Although the present invention has been described with reference to aspecific type of analysis using specific apparatus, upon review of theinformation set forth above that will make numerous modifications andadaptations of the present invention readily apparent to those ofordinary skill in the art. A wide variety of analytical procedures canbe improved by incorporating a variable restrictor into the effluentstream exiting a chromatograph or extraction chamber. Accordingly,reference should be made to the appended claims in order to determinethe full scope of the present invention.

What is claimed is:
 1. Apparatus for separating and classifying therelative amounts of two or more classes of hydrocarbon molecules in asample, comprising: a supercritical fluid chromatograph for receivingthe sample in an input stream and for producing an effluent stream; adetector receiving a first portion of the effluent stream; and avariable orifice restrictor for receiving a second portion of theeffluent stream and for independently controlling the pressure and flowrate of the input stream.
 2. The of apparatus claim 1, wherein thedetector comprises a flame ionization detector.
 3. The apparatus ofclaim 1, further comprising apparatus for identifying the hydrocarbonmolecules in the second effluent stream.
 4. The apparatus of claim 3,wherein the apparatus for identifying the hydrocarbon moleculescomprises an ultraviolet detector.
 5. The apparatus of claim 3, furthercomprising a second detector for characterizing the sample connected toa third effluent stream.
 6. The apparatus of claim 5 wherein the seconddetector comprises a mass spectrometer.
 7. The apparatus of claim 1,wherein the variable restrictor comprises an axially moveable pin torestrict mass flow by varying the size of an orifice.
 8. The apparatusof claim 1, wherein the chromatograph comprises a column packed with anadsorbent silica.
 9. The apparatus of claim 1, wherein the chromatographcomprises a chromatographic column having an internal diameter ofbetween 1 to 5 millimeters (mm).